In Sheet Moulding Composite (SMC) applications, a dispersion or paste is made whereby filler, typically calcium carbonate or alumina trihydrate (ATH) is mixed into a polyester resin. This paste is poured, spread or sprayed onto a carrier film on a conveyor belt, whereupon chopped glass fibres are added. These fibres will provide reinforcement in the final composite. Magnesium oxide powder or paste is added to the mixture, along with peroxides and other additives, followed by a second carrier film and the resulting mixture is pressed through rollers to incorporate the added ingredients. The function of the magnesium oxide is to build a network within the composite, thus increasing the viscosity significantly and rapidly, to a point where the whole mixture is semi-solid. The sheet composite thus formed can then be rolled into a roll (like a rolled rug or carpet) and stored and transported under refrigerated conditions. The end-user of the SMC thus formed may cut sections from the roll to a requisite size, place the piece in a pair of moulds and by the heating and pressure imparted by the mould, the sheet is pressed to the shape of the matched pair of moulds and the composite cross-links to form a rigid fused part or article.
Upon addition of magnesium oxide, the thickening mechanism happens quickly and to a high extent (say from 8000 millipoise to 500,000 millipoise over a few days), so that the time interval between SMC formation and rolling and storage of the sheet is minimized. Acidic dispersing agents, although providing a good reduction of viscosity and good wetting ability when the initial filled paste is formed, can have a deleterious effect upon the subsequent MgO-initiated thickening or maturation stage if they are too strongly acidic. It is believed that a highly acidic dispersing agent anchors to the surface of the magnesium oxide particles, thus preventing the particles forming bonds with the less acidic groups in the unsaturated polyester. It can thus be seen that a dispersing, agent with an optimized level of acidity will present a distinct advantage over a more strongly acidic dispersing agent.
A similar process and advantage of such optimized-acidity dispersing agents may be observed in Bulk Moulding Composites (BMC) where the refrigerated and stored material is in the form of a bulk material such as a ball or block of dough, rather than as a sheet or roll.
There are numerous publications in the patent literature which disclose polyester amine dispersants derived from a poly(C2-4-alkylene imine) such as polyethyleneimine to which is attached a polyester chain. The polyester chain may be derived from 12-hydroxy stearic acid as disclosed in U.S. Pat. No. 4,224,212 or it may be derived from two or more different hydroxy carboxylic acids or lactones thereof as disclosed in U.S. Pat. Nos. 5,700,395 and 6,197,877. These dispersants are effective on inorganic pigments and fillers in general industrial coatings especially coil coating paint formulations. However, they tend to yellow the final paint coating and degrade the gloss/haze values of the final coating. Polyester phosphate dispersants containing similar polyester chains, as disclosed in U.S. Pat. No. 5,300,255, which are attached to phosphate groups tend not to yellow final paint coatings but gloss and haze values of the paint film can be degraded due to the incompatibility of the polyester chains to the paint formulation. Other disadvantages of the polyester amine and polyester phosphate dispersants are that they can suffer from seeding or crystallization in various polar solvents at room temperature. Consequently, acidic dispersants are sought which can disperse an inorganic pigment or filler in an organic medium such as a polar organic liquid and which do not suffer the disadvantages mentioned above.
Dispersants containing terminal acidic groups such as phosphates are known and are generally prepared by reaction of a hydroxy ended polymer chain with phosphorus pentoxide, phosphorus oxychloride, phosphorus trichloride and pentachloride or polyphosphoric acid. The polymer chains are usually polyester or polyalkoxylate chains containing terminal hydroxyl groups.
U.S. Pat. Nos. 5,151,218 and 5,130,463 disclose a dispersant phosphoric acid ester and salts thereof, wherein the ester group is an aliphatic, cycloaliphatic and/or aromatic moiety having no Zerewitinoff hydrogen, containing at least one ether oxygen atom (—O—) and at least one carboxylic acid ester group (—COO—) and/or urethane group (—NCOO—). The dispersant may be used in a number of coating or thermosetting composite applications. U.S. Pat. No. 5,130,463 also discloses dispersants containing a polyether/polyester derived from ε-caprolactone reacted with polyphosphoric acid. The dispersants are useful in a polar medium, such as ketones and esters. U.S. Pat. No. 6,562,897 discloses dispersants containing a polyether/polyester derived from ε-caprolactone and δ-valerolactone reacted with polyphosphoric acid. The dispersants of all three patents are useful in a polar medium, such as ketones and esters.
U.S. Pat. Nos. 5,464,895 and 5,412,139 both disclose polyaryl organophosphate dispersing agents suitable for thermosetting compositions.
U.S. Pat. No. 6,051,627 discloses dispersants including a polyether derived from ethylene oxide and propylene oxide reacted with polyphosphoric acid. The dispersants are useful in a polar medium such as ketones, esters and water.
U.S. Pat. Nos. 2,213,477; 2,454,542; 3,004,056; 3,004,057, 3,010,903, 3,033,889, 3,099,676, 3,235,627; 4,456,485; 4,720,514; 4,872,916; and 5,914,072 all disclose polyalkoxylates containing terminal hydroxyl groups that have been reacted with various phosphorylating agents.
U.S. Pat. No. 3,462,520 discloses the preparation of phosphate esters of linear C8-20 chain polyalkoxylates. References therein disclose the preparation of phosphate esters of ethoxylates of alkylated phenols, ethoxylates of branch chain aliphatic alcohols and ethoxylates of linear primary alcohols of from C10-15 atoms. All of these compositions are used as emulsifiers, wetting agents dispersing agents, detergents and the like in aqueous applications.
U.S. Pat. No. 5,160,725 discloses the preparation of methoxypolyethyleneglycol phosphates and their use as dispersants of magnetic particles such as iron oxide in aqueous solutions.
U.S. Pat. No. 6,500,917 discloses the preparation of methoxypolyethyleneglycol phosphates from the corresponding diaryl or diaralkyl phosphate ester.